The fabrication of mitered joints on wooden workpieces is a very common problem, and especially the cutting of 45.degree. mitered ends to form rectangular structures such as frames for paintings and other pictures, door and window frames, doors, windows and screens, and many other similar end products. It is still, however, a continuing problem, especially for the home workshop operator, to obtain truly accurate 45.degree. cuts on all of the eight workpiece ends which are needed to make a truly rectangular frame or other mitered cornered product.
Probably the most common cause contributing to unsatisfactory mitered joints is that if the saw blade and whatever guide or jig is used therewith do not assure that each cut on each end of each workpiece is accurately at 45.degree. to the length of the workpiece and perpendicular to its surfaces, whatever deviation in accuracy exists will affect each corner of the end product. In other words, the total effect of even very small error will be multiplied by a factor of eight for the end product when each cut is made with the workpiece on the same side of the saw blade.
More specifically, if it is assumed that the saw blade is accurately perpendicular to the associated worktable but that the actual cut is at 45.2.degree. to the length of each workpiece, and that each cut is made with the blade in the same position, then when it is attempted to join the four workpiece to form a rectangular frame, the error at any one corner will be 0.4.degree., the total error at the first three corners will be 1.2.degree., and the fourth corner cannot be completed without warping the frame. This condition is bad enough when the saw blade is accurately perpendicular to the worktable, but it becomes worse when the saw is tilted, whether or not it is set to cut at an accurate 45.degree. to the length of each workpiece.
It is theoretically possible to compensate for such irregularities by making each mating pair of mitered cuts on opposite sides of the saw blade, in which case whatever deviation there may be from the desired accurate cut is compensated for by deviation in the opposite direction of the mating cut on the other workpiece. To carry out this operation, however, is not as simple as it sounds.
In the first place, there are four commonly available ways of making a controlled angular cut on a wooden workpiece. The first is to use a miter box which includes adjustable guides for supporting a hand saw, but this adds a number of difficulties. One is that the saw must neccessarily have a running clearance in the guides, which inherently affects the accuracy of the cut, in both horizontal and vertical planes. For example, the blade may tilt in one or both planes, any such deviation is established as soon as the teeth have firm engagement with the workpiece, and there is no practical way of compensating for the resulting error.
A miter box also requires that the saw guides be set to define the desired angle between the saw blade and the fence against which the workpiece is held during the sawing operation. Even with built-in stops, this angle can readily deviate from a true 45.degree.. This particular type of deviation can be compensated for by cutting the opposite ends of each workpiece on opposite sides of the saw blade provided the workpieces are turned over between cuts. However, this can be done only with workpiece having top and bottom surfaces which are flat and parallel with each other, and it cannot be successfully done with wood having a profiled surface on one side. Otherwise, the saw guides must be moved to their other 45.degree. position, which can also vary in either direction from an accurate 45.degree. setting.
Mitering saw assemblies wherein a power-operated saw is supported on a base for vertical movement offer little improvement over hand-operated miter boxes as discussed above. In such units, the work-supporting table and fence must be adjustable rotationally with respect to the saw blade to establish the angle of cut with respect to the fence. As with the conventional miter box, however, there is no assurance that either of the two 45.degree. positions will be accurate, and the same limitations exist with respect to mating cuts on opposite sides of the saw blade.
Another alternative is a radial arm saw, wherein the power-operated saw blade travels on a horizontal arm above the worktable, and this arm is adjustable to vary the angle between the direction of cut and the fence at the back of the worktable. This type of saw offers all of the same disadvantages and opportunities for error as a powered miter box as discussed above.
The remaining one of the commonly available saw types is a table saw wherein the power operated saw is mounted below the table with its blade projecting upwardly through a slot so that the workpiece can be moved along the table with respect to the blade. Such a saw is usually provided with a miter gauge which is movable along a guide track located on opposite one or both sides of the blade, and which is adjustable to establish the angle defined by the blade and the working face on the gauge against which each workpiece is held as it is moved past the blade.
This type of table saw arrangement still offers the same disadvantages and opportunities for error as the other types of mitering apparatus discussed above. In particular, in order to make miter cuts on profiled workpieces, the adjustment of the miter gauge must be changed, regardless of whether all cuts are made on the same side of the blade or they made alternately on opposite sides of the blade. Any change of adjustment is inherently capable of resulting in error, for the reason already discussed, and there is therefore no greater assurance that the mating cuts will be accurate when they are made on a table saw as when they are made on any of the alternative mitering saw assemblies.